WO 96/248408 describes a process for the production and recovery of dimethyl ether by dehydration of methanol which reduces the distillation duties associated with the preparation of fresh methanol feedstock and/or recovery of unconverted methanol for use as recycle feedstock to the dimethyl ether production process while maintaining a high rate of conversion of methanol to dimethyl ether.
GB 2253623 describes a process for the production of dimethyl ether by feeding synthesis gas in a reactor with a catalyst composition and withdrawing a dimethyl ether-containing effluent wherein carbon dioxide obtained from the reactor effluent is recycled to the reactor.
Processes for the co-production of acetic acid and dimethyl ether may be carried out by catalytic dehydration and hydrolysis of mixtures of methanol and methyl acetate. Such co-production processes are known from, for example WO 2011/027105. WO 2011/027105 describes processes for the co-production of acetic acid and dimethyl ether by contacting methanol and methyl acetate with a catalyst composition at a temperature in the range 140 to 250 C which catalyst composition comprises a zeolite having a 2-dimensional channel system comprising at least one channel which has a 10-membered ring.
In such dehydration-hydrolysis processes methanol is dehydrated to dimethyl ether and methyl acetate is hydrolysed to acetic acid. The reactions can be represented by:2 methanoldimethyl ether+watermethyl acetate+wateracetic acid+methanol
These reactions are equilibrium limited. The hydrolysis reaction consumes water and produces methanol and the dehydration reaction consumes methanol and produces water.
It has now been found that in the presence of solid acid catalysts, such as zeolites, the dehydration reaction is relatively slow and since water is consumed more quickly by the hydrolysis reaction, it is typically necessary to provide water to the system to maintain a steady-state concentration of water in the reaction. Water may be added to the process through process streams such as feed and recycle streams to the dehydration-hydrolysis process.
In general, methanol obtained by commercial synthesis processes contains water and may also contain some dimethyl ether. The amount of water present in produced methanol can vary depending upon such factors as the specific process operating conditions used in the process and the composition of the feed to the methanol synthesis process, and in particular the amount of carbon dioxide employed.
Thus the amount of water present in feeds, particularly in methanol feeds, to dehydration-hydrolysis processes may be sub-optimal for maintaining or optimising the operation of such processes. Furthermore, if such processes are operated as continuous processes, recycling of water-containing streams to the process can cause or contribute to fluctuations in the water concentration within the process.
In commercial chemical processes, process streams for disposal, such as process water streams, comprise or are treated so as to comprise acceptable levels of organic contaminants prior to their disposal. For example, the presence of acetic acid in process water streams for disposal can be problematic. Simple distillation processes can be successfully employed to reduce the concentration of acetic acid in water to about 3 mol % acetic acid but are not effective to reduce the concentration of acetic acid further. Additional processing to meet the demands of economic and/or environment considerations can be employed to reduce the acetic acid concentration to acceptable levels but additional processing, for example by reverse osmosis techniques, is likely to result in a loss of process value and/or higher process operating costs.